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// SPDX-License-Identifier: GPL-2.0+
/*
* RCU CPU stall warnings for normal RCU grace periods
*
* Copyright IBM Corporation , 2019
*
* Author : Paul E . McKenney < paulmck @ linux . ibm . com >
*/
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//////////////////////////////////////////////////////////////////////////////
//
// Controlling CPU stall warnings, including delay calculation.
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/* panic() on RCU Stall sysctl. */
int sysctl_panic_on_rcu_stall __read_mostly ;
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int sysctl_max_rcu_stall_to_panic __read_mostly ;
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# ifdef CONFIG_PROVE_RCU
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# define RCU_STALL_DELAY_DELTA (5 * HZ)
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# else
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# define RCU_STALL_DELAY_DELTA 0
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# endif
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# define RCU_STALL_MIGHT_DIV 8
# define RCU_STALL_MIGHT_MIN (2 * HZ)
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/* Limit-check stall timeouts specified at boottime and runtime. */
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int rcu_jiffies_till_stall_check ( void )
{
int till_stall_check = READ_ONCE ( rcu_cpu_stall_timeout ) ;
/*
* Limit check must be consistent with the Kconfig limits
* for CONFIG_RCU_CPU_STALL_TIMEOUT .
*/
if ( till_stall_check < 3 ) {
WRITE_ONCE ( rcu_cpu_stall_timeout , 3 ) ;
till_stall_check = 3 ;
} else if ( till_stall_check > 300 ) {
WRITE_ONCE ( rcu_cpu_stall_timeout , 300 ) ;
till_stall_check = 300 ;
}
return till_stall_check * HZ + RCU_STALL_DELAY_DELTA ;
}
EXPORT_SYMBOL_GPL ( rcu_jiffies_till_stall_check ) ;
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/**
* rcu_gp_might_be_stalled - Is it likely that the grace period is stalled ?
*
* Returns @ true if the current grace period is sufficiently old that
* it is reasonable to assume that it might be stalled . This can be
* useful when deciding whether to allocate memory to enable RCU - mediated
* freeing on the one hand or just invoking synchronize_rcu ( ) on the other .
* The latter is preferable when the grace period is stalled .
*
* Note that sampling of the . gp_start and . gp_seq fields must be done
* carefully to avoid false positives at the beginnings and ends of
* grace periods .
*/
bool rcu_gp_might_be_stalled ( void )
{
unsigned long d = rcu_jiffies_till_stall_check ( ) / RCU_STALL_MIGHT_DIV ;
unsigned long j = jiffies ;
if ( d < RCU_STALL_MIGHT_MIN )
d = RCU_STALL_MIGHT_MIN ;
smp_mb ( ) ; // jiffies before .gp_seq to avoid false positives.
if ( ! rcu_gp_in_progress ( ) )
return false ;
// Long delays at this point avoids false positive, but a delay
// of ULONG_MAX/4 jiffies voids your no-false-positive warranty.
smp_mb ( ) ; // .gp_seq before second .gp_start
// And ditto here.
return ! time_before ( j , READ_ONCE ( rcu_state . gp_start ) + d ) ;
}
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/* Don't do RCU CPU stall warnings during long sysrq printouts. */
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void rcu_sysrq_start ( void )
{
if ( ! rcu_cpu_stall_suppress )
rcu_cpu_stall_suppress = 2 ;
}
void rcu_sysrq_end ( void )
{
if ( rcu_cpu_stall_suppress = = 2 )
rcu_cpu_stall_suppress = 0 ;
}
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/* Don't print RCU CPU stall warnings during a kernel panic. */
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static int rcu_panic ( struct notifier_block * this , unsigned long ev , void * ptr )
{
rcu_cpu_stall_suppress = 1 ;
return NOTIFY_DONE ;
}
static struct notifier_block rcu_panic_block = {
. notifier_call = rcu_panic ,
} ;
static int __init check_cpu_stall_init ( void )
{
atomic_notifier_chain_register ( & panic_notifier_list , & rcu_panic_block ) ;
return 0 ;
}
early_initcall ( check_cpu_stall_init ) ;
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/* If so specified via sysctl, panic, yielding cleaner stall-warning output. */
static void panic_on_rcu_stall ( void )
{
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static int cpu_stall ;
if ( + + cpu_stall < sysctl_max_rcu_stall_to_panic )
return ;
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if ( sysctl_panic_on_rcu_stall )
panic ( " RCU Stall \n " ) ;
}
/**
* rcu_cpu_stall_reset - prevent further stall warnings in current grace period
*
* Set the stall - warning timeout way off into the future , thus preventing
* any RCU CPU stall - warning messages from appearing in the current set of
* RCU grace periods .
*
* The caller must disable hard irqs .
*/
void rcu_cpu_stall_reset ( void )
{
WRITE_ONCE ( rcu_state . jiffies_stall , jiffies + ULONG_MAX / 2 ) ;
}
//////////////////////////////////////////////////////////////////////////////
//
// Interaction with RCU grace periods
/* Start of new grace period, so record stall time (and forcing times). */
static void record_gp_stall_check_time ( void )
{
unsigned long j = jiffies ;
unsigned long j1 ;
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WRITE_ONCE ( rcu_state . gp_start , j ) ;
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j1 = rcu_jiffies_till_stall_check ( ) ;
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smp_mb ( ) ; // ->gp_start before ->jiffies_stall and caller's ->gp_seq.
WRITE_ONCE ( rcu_state . jiffies_stall , j + j1 ) ;
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rcu_state . jiffies_resched = j + j1 / 2 ;
rcu_state . n_force_qs_gpstart = READ_ONCE ( rcu_state . n_force_qs ) ;
}
/* Zero ->ticks_this_gp and snapshot the number of RCU softirq handlers. */
static void zero_cpu_stall_ticks ( struct rcu_data * rdp )
{
rdp - > ticks_this_gp = 0 ;
rdp - > softirq_snap = kstat_softirqs_cpu ( RCU_SOFTIRQ , smp_processor_id ( ) ) ;
WRITE_ONCE ( rdp - > last_fqs_resched , jiffies ) ;
}
/*
* If too much time has passed in the current grace period , and if
* so configured , go kick the relevant kthreads .
*/
static void rcu_stall_kick_kthreads ( void )
{
unsigned long j ;
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if ( ! READ_ONCE ( rcu_kick_kthreads ) )
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return ;
j = READ_ONCE ( rcu_state . jiffies_kick_kthreads ) ;
if ( time_after ( jiffies , j ) & & rcu_state . gp_kthread & &
( rcu_gp_in_progress ( ) | | READ_ONCE ( rcu_state . gp_flags ) ) ) {
WARN_ONCE ( 1 , " Kicking %s grace-period kthread \n " ,
rcu_state . name ) ;
rcu_ftrace_dump ( DUMP_ALL ) ;
wake_up_process ( rcu_state . gp_kthread ) ;
WRITE_ONCE ( rcu_state . jiffies_kick_kthreads , j + HZ ) ;
}
}
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/*
* Handler for the irq_work request posted about halfway into the RCU CPU
* stall timeout , and used to detect excessive irq disabling . Set state
* appropriately , but just complain if there is unexpected state on entry .
*/
static void rcu_iw_handler ( struct irq_work * iwp )
{
struct rcu_data * rdp ;
struct rcu_node * rnp ;
rdp = container_of ( iwp , struct rcu_data , rcu_iw ) ;
rnp = rdp - > mynode ;
raw_spin_lock_rcu_node ( rnp ) ;
if ( ! WARN_ON_ONCE ( ! rdp - > rcu_iw_pending ) ) {
rdp - > rcu_iw_gp_seq = rnp - > gp_seq ;
rdp - > rcu_iw_pending = false ;
}
raw_spin_unlock_rcu_node ( rnp ) ;
}
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//////////////////////////////////////////////////////////////////////////////
//
// Printing RCU CPU stall warnings
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# ifdef CONFIG_PREEMPT_RCU
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/*
* Dump detailed information for all tasks blocking the current RCU
* grace period on the specified rcu_node structure .
*/
static void rcu_print_detail_task_stall_rnp ( struct rcu_node * rnp )
{
unsigned long flags ;
struct task_struct * t ;
raw_spin_lock_irqsave_rcu_node ( rnp , flags ) ;
if ( ! rcu_preempt_blocked_readers_cgp ( rnp ) ) {
raw_spin_unlock_irqrestore_rcu_node ( rnp , flags ) ;
return ;
}
t = list_entry ( rnp - > gp_tasks - > prev ,
struct task_struct , rcu_node_entry ) ;
list_for_each_entry_continue ( t , & rnp - > blkd_tasks , rcu_node_entry ) {
/*
* We could be printing a lot while holding a spinlock .
* Avoid triggering hard lockup .
*/
touch_nmi_watchdog ( ) ;
sched_show_task ( t ) ;
}
raw_spin_unlock_irqrestore_rcu_node ( rnp , flags ) ;
}
rcu: Add per-task state to RCU CPU stall warnings
Currently, an RCU-preempt CPU stall warning simply lists the PIDs of
those tasks holding up the current grace period. This can be helpful,
but more can be even more helpful.
To this end, this commit adds the nesting level, whether the task
thinks it was preempted in its current RCU read-side critical section,
whether RCU core has asked this task for a quiescent state, whether the
expedited-grace-period hint is set, and whether the task believes that
it is on the blocked-tasks list (it must be, or it would not be printed,
but if things are broken, best not to take too much for granted).
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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// Communicate task state back to the RCU CPU stall warning request.
struct rcu_stall_chk_rdr {
int nesting ;
union rcu_special rs ;
bool on_blkd_list ;
} ;
/*
* Report out the state of a not - running task that is stalling the
* current RCU grace period .
*/
static bool check_slow_task ( struct task_struct * t , void * arg )
{
struct rcu_stall_chk_rdr * rscrp = arg ;
if ( task_curr ( t ) )
return false ; // It is running, so decline to inspect it.
rscrp - > nesting = t - > rcu_read_lock_nesting ;
rscrp - > rs = t - > rcu_read_unlock_special ;
rscrp - > on_blkd_list = ! list_empty ( & t - > rcu_node_entry ) ;
return true ;
}
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/*
* Scan the current list of tasks blocked within RCU read - side critical
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* sections , printing out the tid of each of the first few of them .
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*/
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static int rcu_print_task_stall ( struct rcu_node * rnp , unsigned long flags )
__releases ( rnp - > lock )
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{
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int i = 0 ;
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int ndetected = 0 ;
rcu: Add per-task state to RCU CPU stall warnings
Currently, an RCU-preempt CPU stall warning simply lists the PIDs of
those tasks holding up the current grace period. This can be helpful,
but more can be even more helpful.
To this end, this commit adds the nesting level, whether the task
thinks it was preempted in its current RCU read-side critical section,
whether RCU core has asked this task for a quiescent state, whether the
expedited-grace-period hint is set, and whether the task believes that
it is on the blocked-tasks list (it must be, or it would not be printed,
but if things are broken, best not to take too much for granted).
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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struct rcu_stall_chk_rdr rscr ;
struct task_struct * t ;
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struct task_struct * ts [ 8 ] ;
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lockdep_assert_irqs_disabled ( ) ;
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if ( ! rcu_preempt_blocked_readers_cgp ( rnp ) )
return 0 ;
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pr_err ( " \t Tasks blocked on level-%d rcu_node (CPUs %d-%d): " ,
rnp - > level , rnp - > grplo , rnp - > grphi ) ;
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t = list_entry ( rnp - > gp_tasks - > prev ,
struct task_struct , rcu_node_entry ) ;
list_for_each_entry_continue ( t , & rnp - > blkd_tasks , rcu_node_entry ) {
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get_task_struct ( t ) ;
ts [ i + + ] = t ;
if ( i > = ARRAY_SIZE ( ts ) )
break ;
}
raw_spin_unlock_irqrestore_rcu_node ( rnp , flags ) ;
for ( i - - ; i ; i - - ) {
t = ts [ i ] ;
rcu: Add per-task state to RCU CPU stall warnings
Currently, an RCU-preempt CPU stall warning simply lists the PIDs of
those tasks holding up the current grace period. This can be helpful,
but more can be even more helpful.
To this end, this commit adds the nesting level, whether the task
thinks it was preempted in its current RCU read-side critical section,
whether RCU core has asked this task for a quiescent state, whether the
expedited-grace-period hint is set, and whether the task believes that
it is on the blocked-tasks list (it must be, or it would not be printed,
but if things are broken, best not to take too much for granted).
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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if ( ! try_invoke_on_locked_down_task ( t , check_slow_task , & rscr ) )
pr_cont ( " P%d " , t - > pid ) ;
else
pr_cont ( " P%d/%d:%c%c%c%c " ,
t - > pid , rscr . nesting ,
" .b " [ rscr . rs . b . blocked ] ,
" .q " [ rscr . rs . b . need_qs ] ,
" .e " [ rscr . rs . b . exp_hint ] ,
" .l " [ rscr . on_blkd_list ] ) ;
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lockdep_assert_irqs_disabled ( ) ;
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put_task_struct ( t ) ;
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ndetected + + ;
}
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pr_cont ( " \n " ) ;
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return ndetected ;
}
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# else /* #ifdef CONFIG_PREEMPT_RCU */
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/*
* Because preemptible RCU does not exist , we never have to check for
* tasks blocked within RCU read - side critical sections .
*/
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static void rcu_print_detail_task_stall_rnp ( struct rcu_node * rnp )
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{
}
/*
* Because preemptible RCU does not exist , we never have to check for
* tasks blocked within RCU read - side critical sections .
*/
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static int rcu_print_task_stall ( struct rcu_node * rnp , unsigned long flags )
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{
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raw_spin_unlock_irqrestore_rcu_node ( rnp , flags ) ;
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return 0 ;
}
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# endif /* #else #ifdef CONFIG_PREEMPT_RCU */
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/*
* Dump stacks of all tasks running on stalled CPUs . First try using
* NMIs , but fall back to manual remote stack tracing on architectures
* that don ' t support NMI - based stack dumps . The NMI - triggered stack
* traces are more accurate because they are printed by the target CPU .
*/
static void rcu_dump_cpu_stacks ( void )
{
int cpu ;
unsigned long flags ;
struct rcu_node * rnp ;
rcu_for_each_leaf_node ( rnp ) {
raw_spin_lock_irqsave_rcu_node ( rnp , flags ) ;
for_each_leaf_node_possible_cpu ( rnp , cpu )
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if ( rnp - > qsmask & leaf_node_cpu_bit ( rnp , cpu ) ) {
if ( cpu_is_offline ( cpu ) )
pr_err ( " Offline CPU %d blocking current GP. \n " , cpu ) ;
else if ( ! trigger_single_cpu_backtrace ( cpu ) )
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dump_cpu_task ( cpu ) ;
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}
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raw_spin_unlock_irqrestore_rcu_node ( rnp , flags ) ;
}
}
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# ifdef CONFIG_RCU_FAST_NO_HZ
static void print_cpu_stall_fast_no_hz ( char * cp , int cpu )
{
struct rcu_data * rdp = & per_cpu ( rcu_data , cpu ) ;
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sprintf ( cp , " last_accelerate: %04lx/%04lx dyntick_enabled: %d " ,
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rdp - > last_accelerate & 0xffff , jiffies & 0xffff ,
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! ! rdp - > tick_nohz_enabled_snap ) ;
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}
# else /* #ifdef CONFIG_RCU_FAST_NO_HZ */
static void print_cpu_stall_fast_no_hz ( char * cp , int cpu )
{
* cp = ' \0 ' ;
}
# endif /* #else #ifdef CONFIG_RCU_FAST_NO_HZ */
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static const char * const gp_state_names [ ] = {
[ RCU_GP_IDLE ] = " RCU_GP_IDLE " ,
[ RCU_GP_WAIT_GPS ] = " RCU_GP_WAIT_GPS " ,
[ RCU_GP_DONE_GPS ] = " RCU_GP_DONE_GPS " ,
[ RCU_GP_ONOFF ] = " RCU_GP_ONOFF " ,
[ RCU_GP_INIT ] = " RCU_GP_INIT " ,
[ RCU_GP_WAIT_FQS ] = " RCU_GP_WAIT_FQS " ,
[ RCU_GP_DOING_FQS ] = " RCU_GP_DOING_FQS " ,
[ RCU_GP_CLEANUP ] = " RCU_GP_CLEANUP " ,
[ RCU_GP_CLEANED ] = " RCU_GP_CLEANED " ,
} ;
/*
* Convert a - > gp_state value to a character string .
*/
static const char * gp_state_getname ( short gs )
{
if ( gs < 0 | | gs > = ARRAY_SIZE ( gp_state_names ) )
return " ??? " ;
return gp_state_names [ gs ] ;
}
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/* Is the RCU grace-period kthread being starved of CPU time? */
static bool rcu_is_gp_kthread_starving ( unsigned long * jp )
{
unsigned long j = jiffies - READ_ONCE ( rcu_state . gp_activity ) ;
if ( jp )
* jp = j ;
return j > 2 * HZ ;
}
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/*
* Print out diagnostic information for the specified stalled CPU .
*
* If the specified CPU is aware of the current RCU grace period , then
* print the number of scheduling clock interrupts the CPU has taken
* during the time that it has been aware . Otherwise , print the number
* of RCU grace periods that this CPU is ignorant of , for example , " 1 "
* if the CPU was aware of the previous grace period .
*
* Also print out idle and ( if CONFIG_RCU_FAST_NO_HZ ) idle - entry info .
*/
static void print_cpu_stall_info ( int cpu )
{
unsigned long delta ;
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bool falsepositive ;
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char fast_no_hz [ 72 ] ;
struct rcu_data * rdp = per_cpu_ptr ( & rcu_data , cpu ) ;
char * ticks_title ;
unsigned long ticks_value ;
/*
* We could be printing a lot while holding a spinlock . Avoid
* triggering hard lockup .
*/
touch_nmi_watchdog ( ) ;
ticks_value = rcu_seq_ctr ( rcu_state . gp_seq - rdp - > gp_seq ) ;
if ( ticks_value ) {
ticks_title = " GPs behind " ;
} else {
ticks_title = " ticks this GP " ;
ticks_value = rdp - > ticks_this_gp ;
}
print_cpu_stall_fast_no_hz ( fast_no_hz , cpu ) ;
delta = rcu_seq_ctr ( rdp - > mynode - > gp_seq - rdp - > rcu_iw_gp_seq ) ;
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falsepositive = rcu_is_gp_kthread_starving ( NULL ) & &
rcu_dynticks_in_eqs ( rcu_dynticks_snap ( rdp ) ) ;
pr_err ( " \t %d-%c%c%c%c: (%lu %s) idle=%03x/%ld/%#lx softirq=%u/%u fqs=%ld %s%s \n " ,
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cpu ,
" O. " [ ! ! cpu_online ( cpu ) ] ,
" o. " [ ! ! ( rdp - > grpmask & rdp - > mynode - > qsmaskinit ) ] ,
" N. " [ ! ! ( rdp - > grpmask & rdp - > mynode - > qsmaskinitnext ) ] ,
! IS_ENABLED ( CONFIG_IRQ_WORK ) ? ' ? ' :
rdp - > rcu_iw_pending ? ( int ) min ( delta , 9UL ) + ' 0 ' :
" !. " [ ! delta ] ,
ticks_value , ticks_title ,
rcu_dynticks_snap ( rdp ) & 0xfff ,
rdp - > dynticks_nesting , rdp - > dynticks_nmi_nesting ,
rdp - > softirq_snap , kstat_softirqs_cpu ( RCU_SOFTIRQ , cpu ) ,
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data_race ( rcu_state . n_force_qs ) - rcu_state . n_force_qs_gpstart ,
fast_no_hz ,
falsepositive ? " (false positive?) " : " " ) ;
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}
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/* Complain about starvation of grace-period kthread. */
static void rcu_check_gp_kthread_starvation ( void )
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{
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int cpu ;
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struct task_struct * gpk = rcu_state . gp_kthread ;
unsigned long j ;
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if ( rcu_is_gp_kthread_starving ( & j ) ) {
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cpu = gpk ? task_cpu ( gpk ) : - 1 ;
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pr_err ( " %s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#lx ->cpu=%d \n " ,
rcu_state . name , j ,
( long ) rcu_seq_current ( & rcu_state . gp_seq ) ,
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data_race ( rcu_state . gp_flags ) ,
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gp_state_getname ( rcu_state . gp_state ) , rcu_state . gp_state ,
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gpk ? gpk - > state : ~ 0 , cpu ) ;
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if ( gpk ) {
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pr_err ( " \t Unless %s kthread gets sufficient CPU time, OOM is now expected behavior. \n " , rcu_state . name ) ;
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pr_err ( " RCU grace-period kthread stack dump: \n " ) ;
sched_show_task ( gpk ) ;
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if ( cpu > = 0 ) {
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if ( cpu_is_offline ( cpu ) ) {
pr_err ( " RCU GP kthread last ran on offline CPU %d. \n " , cpu ) ;
} else {
pr_err ( " Stack dump where RCU GP kthread last ran: \n " ) ;
if ( ! trigger_single_cpu_backtrace ( cpu ) )
dump_cpu_task ( cpu ) ;
}
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}
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wake_up_process ( gpk ) ;
}
}
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}
rcu: Check and report missed fqs timer wakeup on RCU stall
For a new grace period request, the RCU GP kthread transitions through
following states:
a. [RCU_GP_WAIT_GPS] -> [RCU_GP_DONE_GPS]
The RCU_GP_WAIT_GPS state is where the GP kthread waits for a request
for a new GP. Once it receives a request (for example, when a new RCU
callback is queued), the GP kthread transitions to RCU_GP_DONE_GPS.
b. [RCU_GP_DONE_GPS] -> [RCU_GP_ONOFF]
Grace period initialization starts in rcu_gp_init(), which records the
start of new GP in rcu_state.gp_seq and transitions to RCU_GP_ONOFF.
c. [RCU_GP_ONOFF] -> [RCU_GP_INIT]
The purpose of the RCU_GP_ONOFF state is to apply the online/offline
information that was buffered for any CPUs that recently came online or
went offline. This state is maintained in per-leaf rcu_node bitmasks,
with the buffered state in ->qsmaskinitnext and the state for the upcoming
GP in ->qsmaskinit. At the end of this RCU_GP_ONOFF state, each bit in
->qsmaskinit will correspond to a CPU that must pass through a quiescent
state before the upcoming grace period is allowed to complete.
However, a leaf rcu_node structure with an all-zeroes ->qsmaskinit
cannot necessarily be ignored. In preemptible RCU, there might well be
tasks still in RCU read-side critical sections that were first preempted
while running on one of the CPUs managed by this structure. Such tasks
will be queued on this structure's ->blkd_tasks list. Only after this
list fully drains can this leaf rcu_node structure be ignored, and even
then only if none of its CPUs have come back online in the meantime.
Once that happens, the ->qsmaskinit masks further up the tree will be
updated to exclude this leaf rcu_node structure.
Once the ->qsmaskinitnext and ->qsmaskinit fields have been updated
as needed, the GP kthread transitions to RCU_GP_INIT.
d. [RCU_GP_INIT] -> [RCU_GP_WAIT_FQS]
The purpose of the RCU_GP_INIT state is to copy each ->qsmaskinit to
the ->qsmask field within each rcu_node structure. This copying is done
breadth-first from the root to the leaves. Why not just copy directly
from ->qsmaskinitnext to ->qsmask? Because the ->qsmaskinitnext masks
can change in the meantime as additional CPUs come online or go offline.
Such changes would result in inconsistencies in the ->qsmask fields up and
down the tree, which could in turn result in too-short grace periods or
grace-period hangs. These issues are avoided by snapshotting the leaf
rcu_node structures' ->qsmaskinitnext fields into their ->qsmaskinit
counterparts, generating a consistent set of ->qsmaskinit fields
throughout the tree, and only then copying these consistent ->qsmaskinit
fields to their ->qsmask counterparts.
Once this initialization step is complete, the GP kthread transitions
to RCU_GP_WAIT_FQS, where it waits to do a force-quiescent-state scan
on the one hand or for the end of the grace period on the other.
e. [RCU_GP_WAIT_FQS] -> [RCU_GP_DOING_FQS]
The RCU_GP_WAIT_FQS state waits for one of three things: (1) An
explicit request to do a force-quiescent-state scan, (2) The end of
the grace period, or (3) A short interval of time, after which it
will do a force-quiescent-state (FQS) scan. The explicit request can
come from rcutorture or from any CPU that has too many RCU callbacks
queued (see the qhimark kernel parameter and the RCU_GP_FLAG_OVLD
flag). The aforementioned "short period of time" is specified by the
jiffies_till_first_fqs boot parameter for a given grace period's first
FQS scan and by the jiffies_till_next_fqs for later FQS scans.
Either way, once the wait is over, the GP kthread transitions to
RCU_GP_DOING_FQS.
f. [RCU_GP_DOING_FQS] -> [RCU_GP_CLEANUP]
The RCU_GP_DOING_FQS state performs an FQS scan. Each such scan carries
out two functions for any CPU whose bit is still set in its leaf rcu_node
structure's ->qsmask field, that is, for any CPU that has not yet reported
a quiescent state for the current grace period:
i. Report quiescent states on behalf of CPUs that have been observed
to be idle (from an RCU perspective) since the beginning of the
grace period.
ii. If the current grace period is too old, take various actions to
encourage holdout CPUs to pass through quiescent states, including
enlisting the aid of any calls to cond_resched() and might_sleep(),
and even including IPIing the holdout CPUs.
These checks are skipped for any leaf rcu_node structure with a all-zero
->qsmask field, however such structures are subject to RCU priority
boosting if there are tasks on a given structure blocking the current
grace period. The end of the grace period is detected when the root
rcu_node structure's ->qsmask is zero and when there are no longer any
preempted tasks blocking the current grace period. (No, this last check
is not redundant. To see this, consider an rcu_node tree having exactly
one structure that serves as both root and leaf.)
Once the end of the grace period is detected, the GP kthread transitions
to RCU_GP_CLEANUP.
g. [RCU_GP_CLEANUP] -> [RCU_GP_CLEANED]
The RCU_GP_CLEANUP state marks the end of grace period by updating the
rcu_state structure's ->gp_seq field and also all rcu_node structures'
->gp_seq field. As before, the rcu_node tree is traversed in breadth
first order. Once this update is complete, the GP kthread transitions
to the RCU_GP_CLEANED state.
i. [RCU_GP_CLEANED] -> [RCU_GP_INIT]
Once in the RCU_GP_CLEANED state, the GP kthread immediately transitions
into the RCU_GP_INIT state.
j. The role of timers.
If there is at least one idle CPU, and if timers are not firing, the
transition from RCU_GP_DOING_FQS to RCU_GP_CLEANUP will never happen.
Timers can fail to fire for a number of reasons, including issues in
timer configuration, issues in the timer framework, and failure to handle
softirqs (for example, when there is a storm of interrupts). Whatever the
reason, if the timers fail to fire, the GP kthread will never be awakened,
resulting in RCU CPU stall warnings and eventually in OOM.
However, an RCU CPU stall warning has a large number of potential causes,
as documented in Documentation/RCU/stallwarn.rst. This commit therefore
adds analysis to the RCU CPU stall-warning code to emit an additional
message if the cause of the stall is likely to be timer failure.
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2020-11-16 19:06:00 +03:00
/* Complain about missing wakeups from expired fqs wait timer */
static void rcu_check_gp_kthread_expired_fqs_timer ( void )
{
struct task_struct * gpk = rcu_state . gp_kthread ;
short gp_state ;
unsigned long jiffies_fqs ;
int cpu ;
/*
* Order reads of . gp_state and . jiffies_force_qs .
* Matching smp_wmb ( ) is present in rcu_gp_fqs_loop ( ) .
*/
gp_state = smp_load_acquire ( & rcu_state . gp_state ) ;
jiffies_fqs = READ_ONCE ( rcu_state . jiffies_force_qs ) ;
if ( gp_state = = RCU_GP_WAIT_FQS & &
time_after ( jiffies , jiffies_fqs + RCU_STALL_MIGHT_MIN ) & &
gpk & & ! READ_ONCE ( gpk - > on_rq ) ) {
cpu = task_cpu ( gpk ) ;
pr_err ( " %s kthread timer wakeup didn't happen for %ld jiffies! g%ld f%#x %s(%d) ->state=%#lx \n " ,
rcu_state . name , ( jiffies - jiffies_fqs ) ,
( long ) rcu_seq_current ( & rcu_state . gp_seq ) ,
data_race ( rcu_state . gp_flags ) ,
gp_state_getname ( RCU_GP_WAIT_FQS ) , RCU_GP_WAIT_FQS ,
gpk - > state ) ;
pr_err ( " \t Possible timer handling issue on cpu=%d timer-softirq=%u \n " ,
cpu , kstat_softirqs_cpu ( TIMER_SOFTIRQ , cpu ) ) ;
}
}
2020-03-06 01:56:11 +03:00
static void print_other_cpu_stall ( unsigned long gp_seq , unsigned long gps )
2019-01-12 03:57:41 +03:00
{
int cpu ;
unsigned long flags ;
unsigned long gpa ;
unsigned long j ;
int ndetected = 0 ;
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struct rcu_node * rnp ;
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long totqlen = 0 ;
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lockdep_assert_irqs_disabled ( ) ;
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/* Kick and suppress, if so configured. */
rcu_stall_kick_kthreads ( ) ;
2019-12-05 22:29:01 +03:00
if ( rcu_stall_is_suppressed ( ) )
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return ;
/*
* OK , time to rat on our buddy . . .
2020-04-21 20:04:10 +03:00
* See Documentation / RCU / stallwarn . rst for info on how to debug
2019-01-12 03:57:41 +03:00
* RCU CPU stall warnings .
*/
2021-03-02 14:55:15 +03:00
trace_rcu_stall_warning ( rcu_state . name , TPS ( " StallDetected " ) ) ;
2019-01-12 07:58:58 +03:00
pr_err ( " INFO: %s detected stalls on CPUs/tasks: \n " , rcu_state . name ) ;
2019-01-12 03:57:41 +03:00
rcu_for_each_leaf_node ( rnp ) {
raw_spin_lock_irqsave_rcu_node ( rnp , flags ) ;
if ( rnp - > qsmask ! = 0 ) {
for_each_leaf_node_possible_cpu ( rnp , cpu )
if ( rnp - > qsmask & leaf_node_cpu_bit ( rnp , cpu ) ) {
print_cpu_stall_info ( cpu ) ;
ndetected + + ;
}
}
2020-09-25 01:11:55 +03:00
ndetected + = rcu_print_task_stall ( rnp , flags ) ; // Releases rnp->lock.
2020-11-19 21:13:06 +03:00
lockdep_assert_irqs_disabled ( ) ;
2019-01-12 03:57:41 +03:00
}
for_each_possible_cpu ( cpu )
totqlen + = rcu_get_n_cbs_cpu ( cpu ) ;
2019-01-12 07:58:58 +03:00
pr_cont ( " \t (detected by %d, t=%ld jiffies, g=%ld, q=%lu) \n " ,
2020-03-06 01:56:11 +03:00
smp_processor_id ( ) , ( long ) ( jiffies - gps ) ,
2019-01-12 03:57:41 +03:00
( long ) rcu_seq_current ( & rcu_state . gp_seq ) , totqlen ) ;
if ( ndetected ) {
rcu_dump_cpu_stacks ( ) ;
/* Complain about tasks blocking the grace period. */
2019-01-12 07:36:45 +03:00
rcu_for_each_leaf_node ( rnp )
rcu_print_detail_task_stall_rnp ( rnp ) ;
2019-01-12 03:57:41 +03:00
} else {
if ( rcu_seq_current ( & rcu_state . gp_seq ) ! = gp_seq ) {
pr_err ( " INFO: Stall ended before state dump start \n " ) ;
} else {
j = jiffies ;
2020-02-09 13:29:36 +03:00
gpa = data_race ( rcu_state . gp_activity ) ;
2019-01-12 03:57:41 +03:00
pr_err ( " All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx \n " ,
rcu_state . name , j - gpa , j , gpa ,
2020-02-09 13:29:36 +03:00
data_race ( jiffies_till_next_fqs ) ,
2019-01-12 03:57:41 +03:00
rcu_get_root ( ) - > qsmask ) ;
}
}
/* Rewrite if needed in case of slow consoles. */
if ( ULONG_CMP_GE ( jiffies , READ_ONCE ( rcu_state . jiffies_stall ) ) )
WRITE_ONCE ( rcu_state . jiffies_stall ,
jiffies + 3 * rcu_jiffies_till_stall_check ( ) + 3 ) ;
rcu: Check and report missed fqs timer wakeup on RCU stall
For a new grace period request, the RCU GP kthread transitions through
following states:
a. [RCU_GP_WAIT_GPS] -> [RCU_GP_DONE_GPS]
The RCU_GP_WAIT_GPS state is where the GP kthread waits for a request
for a new GP. Once it receives a request (for example, when a new RCU
callback is queued), the GP kthread transitions to RCU_GP_DONE_GPS.
b. [RCU_GP_DONE_GPS] -> [RCU_GP_ONOFF]
Grace period initialization starts in rcu_gp_init(), which records the
start of new GP in rcu_state.gp_seq and transitions to RCU_GP_ONOFF.
c. [RCU_GP_ONOFF] -> [RCU_GP_INIT]
The purpose of the RCU_GP_ONOFF state is to apply the online/offline
information that was buffered for any CPUs that recently came online or
went offline. This state is maintained in per-leaf rcu_node bitmasks,
with the buffered state in ->qsmaskinitnext and the state for the upcoming
GP in ->qsmaskinit. At the end of this RCU_GP_ONOFF state, each bit in
->qsmaskinit will correspond to a CPU that must pass through a quiescent
state before the upcoming grace period is allowed to complete.
However, a leaf rcu_node structure with an all-zeroes ->qsmaskinit
cannot necessarily be ignored. In preemptible RCU, there might well be
tasks still in RCU read-side critical sections that were first preempted
while running on one of the CPUs managed by this structure. Such tasks
will be queued on this structure's ->blkd_tasks list. Only after this
list fully drains can this leaf rcu_node structure be ignored, and even
then only if none of its CPUs have come back online in the meantime.
Once that happens, the ->qsmaskinit masks further up the tree will be
updated to exclude this leaf rcu_node structure.
Once the ->qsmaskinitnext and ->qsmaskinit fields have been updated
as needed, the GP kthread transitions to RCU_GP_INIT.
d. [RCU_GP_INIT] -> [RCU_GP_WAIT_FQS]
The purpose of the RCU_GP_INIT state is to copy each ->qsmaskinit to
the ->qsmask field within each rcu_node structure. This copying is done
breadth-first from the root to the leaves. Why not just copy directly
from ->qsmaskinitnext to ->qsmask? Because the ->qsmaskinitnext masks
can change in the meantime as additional CPUs come online or go offline.
Such changes would result in inconsistencies in the ->qsmask fields up and
down the tree, which could in turn result in too-short grace periods or
grace-period hangs. These issues are avoided by snapshotting the leaf
rcu_node structures' ->qsmaskinitnext fields into their ->qsmaskinit
counterparts, generating a consistent set of ->qsmaskinit fields
throughout the tree, and only then copying these consistent ->qsmaskinit
fields to their ->qsmask counterparts.
Once this initialization step is complete, the GP kthread transitions
to RCU_GP_WAIT_FQS, where it waits to do a force-quiescent-state scan
on the one hand or for the end of the grace period on the other.
e. [RCU_GP_WAIT_FQS] -> [RCU_GP_DOING_FQS]
The RCU_GP_WAIT_FQS state waits for one of three things: (1) An
explicit request to do a force-quiescent-state scan, (2) The end of
the grace period, or (3) A short interval of time, after which it
will do a force-quiescent-state (FQS) scan. The explicit request can
come from rcutorture or from any CPU that has too many RCU callbacks
queued (see the qhimark kernel parameter and the RCU_GP_FLAG_OVLD
flag). The aforementioned "short period of time" is specified by the
jiffies_till_first_fqs boot parameter for a given grace period's first
FQS scan and by the jiffies_till_next_fqs for later FQS scans.
Either way, once the wait is over, the GP kthread transitions to
RCU_GP_DOING_FQS.
f. [RCU_GP_DOING_FQS] -> [RCU_GP_CLEANUP]
The RCU_GP_DOING_FQS state performs an FQS scan. Each such scan carries
out two functions for any CPU whose bit is still set in its leaf rcu_node
structure's ->qsmask field, that is, for any CPU that has not yet reported
a quiescent state for the current grace period:
i. Report quiescent states on behalf of CPUs that have been observed
to be idle (from an RCU perspective) since the beginning of the
grace period.
ii. If the current grace period is too old, take various actions to
encourage holdout CPUs to pass through quiescent states, including
enlisting the aid of any calls to cond_resched() and might_sleep(),
and even including IPIing the holdout CPUs.
These checks are skipped for any leaf rcu_node structure with a all-zero
->qsmask field, however such structures are subject to RCU priority
boosting if there are tasks on a given structure blocking the current
grace period. The end of the grace period is detected when the root
rcu_node structure's ->qsmask is zero and when there are no longer any
preempted tasks blocking the current grace period. (No, this last check
is not redundant. To see this, consider an rcu_node tree having exactly
one structure that serves as both root and leaf.)
Once the end of the grace period is detected, the GP kthread transitions
to RCU_GP_CLEANUP.
g. [RCU_GP_CLEANUP] -> [RCU_GP_CLEANED]
The RCU_GP_CLEANUP state marks the end of grace period by updating the
rcu_state structure's ->gp_seq field and also all rcu_node structures'
->gp_seq field. As before, the rcu_node tree is traversed in breadth
first order. Once this update is complete, the GP kthread transitions
to the RCU_GP_CLEANED state.
i. [RCU_GP_CLEANED] -> [RCU_GP_INIT]
Once in the RCU_GP_CLEANED state, the GP kthread immediately transitions
into the RCU_GP_INIT state.
j. The role of timers.
If there is at least one idle CPU, and if timers are not firing, the
transition from RCU_GP_DOING_FQS to RCU_GP_CLEANUP will never happen.
Timers can fail to fire for a number of reasons, including issues in
timer configuration, issues in the timer framework, and failure to handle
softirqs (for example, when there is a storm of interrupts). Whatever the
reason, if the timers fail to fire, the GP kthread will never be awakened,
resulting in RCU CPU stall warnings and eventually in OOM.
However, an RCU CPU stall warning has a large number of potential causes,
as documented in Documentation/RCU/stallwarn.rst. This commit therefore
adds analysis to the RCU CPU stall-warning code to emit an additional
message if the cause of the stall is likely to be timer failure.
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2020-11-16 19:06:00 +03:00
rcu_check_gp_kthread_expired_fqs_timer ( ) ;
2019-01-12 03:57:41 +03:00
rcu_check_gp_kthread_starvation ( ) ;
panic_on_rcu_stall ( ) ;
rcu_force_quiescent_state ( ) ; /* Kick them all. */
}
2020-03-06 01:56:11 +03:00
static void print_cpu_stall ( unsigned long gps )
2019-01-12 03:57:41 +03:00
{
int cpu ;
unsigned long flags ;
struct rcu_data * rdp = this_cpu_ptr ( & rcu_data ) ;
struct rcu_node * rnp = rcu_get_root ( ) ;
long totqlen = 0 ;
2020-11-19 21:13:06 +03:00
lockdep_assert_irqs_disabled ( ) ;
2019-01-12 03:57:41 +03:00
/* Kick and suppress, if so configured. */
rcu_stall_kick_kthreads ( ) ;
2019-12-05 22:29:01 +03:00
if ( rcu_stall_is_suppressed ( ) )
2019-01-12 03:57:41 +03:00
return ;
/*
* OK , time to rat on ourselves . . .
2020-04-21 20:04:10 +03:00
* See Documentation / RCU / stallwarn . rst for info on how to debug
2019-01-12 03:57:41 +03:00
* RCU CPU stall warnings .
*/
2021-03-02 14:55:15 +03:00
trace_rcu_stall_warning ( rcu_state . name , TPS ( " SelfDetected " ) ) ;
2019-01-12 07:58:58 +03:00
pr_err ( " INFO: %s self-detected stall on CPU \n " , rcu_state . name ) ;
2019-01-12 03:57:41 +03:00
raw_spin_lock_irqsave_rcu_node ( rdp - > mynode , flags ) ;
print_cpu_stall_info ( smp_processor_id ( ) ) ;
raw_spin_unlock_irqrestore_rcu_node ( rdp - > mynode , flags ) ;
for_each_possible_cpu ( cpu )
totqlen + = rcu_get_n_cbs_cpu ( cpu ) ;
2019-01-12 07:58:58 +03:00
pr_cont ( " \t (t=%lu jiffies g=%ld q=%lu) \n " ,
2020-03-06 01:56:11 +03:00
jiffies - gps ,
2019-01-12 03:57:41 +03:00
( long ) rcu_seq_current ( & rcu_state . gp_seq ) , totqlen ) ;
rcu: Check and report missed fqs timer wakeup on RCU stall
For a new grace period request, the RCU GP kthread transitions through
following states:
a. [RCU_GP_WAIT_GPS] -> [RCU_GP_DONE_GPS]
The RCU_GP_WAIT_GPS state is where the GP kthread waits for a request
for a new GP. Once it receives a request (for example, when a new RCU
callback is queued), the GP kthread transitions to RCU_GP_DONE_GPS.
b. [RCU_GP_DONE_GPS] -> [RCU_GP_ONOFF]
Grace period initialization starts in rcu_gp_init(), which records the
start of new GP in rcu_state.gp_seq and transitions to RCU_GP_ONOFF.
c. [RCU_GP_ONOFF] -> [RCU_GP_INIT]
The purpose of the RCU_GP_ONOFF state is to apply the online/offline
information that was buffered for any CPUs that recently came online or
went offline. This state is maintained in per-leaf rcu_node bitmasks,
with the buffered state in ->qsmaskinitnext and the state for the upcoming
GP in ->qsmaskinit. At the end of this RCU_GP_ONOFF state, each bit in
->qsmaskinit will correspond to a CPU that must pass through a quiescent
state before the upcoming grace period is allowed to complete.
However, a leaf rcu_node structure with an all-zeroes ->qsmaskinit
cannot necessarily be ignored. In preemptible RCU, there might well be
tasks still in RCU read-side critical sections that were first preempted
while running on one of the CPUs managed by this structure. Such tasks
will be queued on this structure's ->blkd_tasks list. Only after this
list fully drains can this leaf rcu_node structure be ignored, and even
then only if none of its CPUs have come back online in the meantime.
Once that happens, the ->qsmaskinit masks further up the tree will be
updated to exclude this leaf rcu_node structure.
Once the ->qsmaskinitnext and ->qsmaskinit fields have been updated
as needed, the GP kthread transitions to RCU_GP_INIT.
d. [RCU_GP_INIT] -> [RCU_GP_WAIT_FQS]
The purpose of the RCU_GP_INIT state is to copy each ->qsmaskinit to
the ->qsmask field within each rcu_node structure. This copying is done
breadth-first from the root to the leaves. Why not just copy directly
from ->qsmaskinitnext to ->qsmask? Because the ->qsmaskinitnext masks
can change in the meantime as additional CPUs come online or go offline.
Such changes would result in inconsistencies in the ->qsmask fields up and
down the tree, which could in turn result in too-short grace periods or
grace-period hangs. These issues are avoided by snapshotting the leaf
rcu_node structures' ->qsmaskinitnext fields into their ->qsmaskinit
counterparts, generating a consistent set of ->qsmaskinit fields
throughout the tree, and only then copying these consistent ->qsmaskinit
fields to their ->qsmask counterparts.
Once this initialization step is complete, the GP kthread transitions
to RCU_GP_WAIT_FQS, where it waits to do a force-quiescent-state scan
on the one hand or for the end of the grace period on the other.
e. [RCU_GP_WAIT_FQS] -> [RCU_GP_DOING_FQS]
The RCU_GP_WAIT_FQS state waits for one of three things: (1) An
explicit request to do a force-quiescent-state scan, (2) The end of
the grace period, or (3) A short interval of time, after which it
will do a force-quiescent-state (FQS) scan. The explicit request can
come from rcutorture or from any CPU that has too many RCU callbacks
queued (see the qhimark kernel parameter and the RCU_GP_FLAG_OVLD
flag). The aforementioned "short period of time" is specified by the
jiffies_till_first_fqs boot parameter for a given grace period's first
FQS scan and by the jiffies_till_next_fqs for later FQS scans.
Either way, once the wait is over, the GP kthread transitions to
RCU_GP_DOING_FQS.
f. [RCU_GP_DOING_FQS] -> [RCU_GP_CLEANUP]
The RCU_GP_DOING_FQS state performs an FQS scan. Each such scan carries
out two functions for any CPU whose bit is still set in its leaf rcu_node
structure's ->qsmask field, that is, for any CPU that has not yet reported
a quiescent state for the current grace period:
i. Report quiescent states on behalf of CPUs that have been observed
to be idle (from an RCU perspective) since the beginning of the
grace period.
ii. If the current grace period is too old, take various actions to
encourage holdout CPUs to pass through quiescent states, including
enlisting the aid of any calls to cond_resched() and might_sleep(),
and even including IPIing the holdout CPUs.
These checks are skipped for any leaf rcu_node structure with a all-zero
->qsmask field, however such structures are subject to RCU priority
boosting if there are tasks on a given structure blocking the current
grace period. The end of the grace period is detected when the root
rcu_node structure's ->qsmask is zero and when there are no longer any
preempted tasks blocking the current grace period. (No, this last check
is not redundant. To see this, consider an rcu_node tree having exactly
one structure that serves as both root and leaf.)
Once the end of the grace period is detected, the GP kthread transitions
to RCU_GP_CLEANUP.
g. [RCU_GP_CLEANUP] -> [RCU_GP_CLEANED]
The RCU_GP_CLEANUP state marks the end of grace period by updating the
rcu_state structure's ->gp_seq field and also all rcu_node structures'
->gp_seq field. As before, the rcu_node tree is traversed in breadth
first order. Once this update is complete, the GP kthread transitions
to the RCU_GP_CLEANED state.
i. [RCU_GP_CLEANED] -> [RCU_GP_INIT]
Once in the RCU_GP_CLEANED state, the GP kthread immediately transitions
into the RCU_GP_INIT state.
j. The role of timers.
If there is at least one idle CPU, and if timers are not firing, the
transition from RCU_GP_DOING_FQS to RCU_GP_CLEANUP will never happen.
Timers can fail to fire for a number of reasons, including issues in
timer configuration, issues in the timer framework, and failure to handle
softirqs (for example, when there is a storm of interrupts). Whatever the
reason, if the timers fail to fire, the GP kthread will never be awakened,
resulting in RCU CPU stall warnings and eventually in OOM.
However, an RCU CPU stall warning has a large number of potential causes,
as documented in Documentation/RCU/stallwarn.rst. This commit therefore
adds analysis to the RCU CPU stall-warning code to emit an additional
message if the cause of the stall is likely to be timer failure.
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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rcu_check_gp_kthread_expired_fqs_timer ( ) ;
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rcu_check_gp_kthread_starvation ( ) ;
rcu_dump_cpu_stacks ( ) ;
raw_spin_lock_irqsave_rcu_node ( rnp , flags ) ;
/* Rewrite if needed in case of slow consoles. */
if ( ULONG_CMP_GE ( jiffies , READ_ONCE ( rcu_state . jiffies_stall ) ) )
WRITE_ONCE ( rcu_state . jiffies_stall ,
jiffies + 3 * rcu_jiffies_till_stall_check ( ) + 3 ) ;
raw_spin_unlock_irqrestore_rcu_node ( rnp , flags ) ;
panic_on_rcu_stall ( ) ;
/*
* Attempt to revive the RCU machinery by forcing a context switch .
*
* A context switch would normally allow the RCU state machine to make
* progress and it could be we ' re stuck in kernel space without context
* switches for an entirely unreasonable amount of time .
*/
set_tsk_need_resched ( current ) ;
set_preempt_need_resched ( ) ;
}
static void check_cpu_stall ( struct rcu_data * rdp )
{
unsigned long gs1 ;
unsigned long gs2 ;
unsigned long gps ;
unsigned long j ;
unsigned long jn ;
unsigned long js ;
struct rcu_node * rnp ;
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lockdep_assert_irqs_disabled ( ) ;
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if ( ( rcu_stall_is_suppressed ( ) & & ! READ_ONCE ( rcu_kick_kthreads ) ) | |
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! rcu_gp_in_progress ( ) )
return ;
rcu_stall_kick_kthreads ( ) ;
j = jiffies ;
/*
* Lots of memory barriers to reject false positives .
*
* The idea is to pick up rcu_state . gp_seq , then
* rcu_state . jiffies_stall , then rcu_state . gp_start , and finally
* another copy of rcu_state . gp_seq . These values are updated in
* the opposite order with memory barriers ( or equivalent ) during
* grace - period initialization and cleanup . Now , a false positive
* can occur if we get an new value of rcu_state . gp_start and a old
* value of rcu_state . jiffies_stall . But given the memory barriers ,
* the only way that this can happen is if one grace period ends
* and another starts between these two fetches . This is detected
* by comparing the second fetch of rcu_state . gp_seq with the
* previous fetch from rcu_state . gp_seq .
*
* Given this check , comparisons of jiffies , rcu_state . jiffies_stall ,
* and rcu_state . gp_start suffice to forestall false positives .
*/
gs1 = READ_ONCE ( rcu_state . gp_seq ) ;
smp_rmb ( ) ; /* Pick up ->gp_seq first... */
js = READ_ONCE ( rcu_state . jiffies_stall ) ;
smp_rmb ( ) ; /* ...then ->jiffies_stall before the rest... */
gps = READ_ONCE ( rcu_state . gp_start ) ;
smp_rmb ( ) ; /* ...and finally ->gp_start before ->gp_seq again. */
gs2 = READ_ONCE ( rcu_state . gp_seq ) ;
if ( gs1 ! = gs2 | |
ULONG_CMP_LT ( j , js ) | |
ULONG_CMP_GE ( gps , js ) )
return ; /* No stall or GP completed since entering function. */
rnp = rdp - > mynode ;
jn = jiffies + 3 * rcu_jiffies_till_stall_check ( ) + 3 ;
if ( rcu_gp_in_progress ( ) & &
( READ_ONCE ( rnp - > qsmask ) & rdp - > grpmask ) & &
cmpxchg ( & rcu_state . jiffies_stall , js , jn ) = = js ) {
/* We haven't checked in, so go dump stack. */
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print_cpu_stall ( gps ) ;
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if ( READ_ONCE ( rcu_cpu_stall_ftrace_dump ) )
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rcu_ftrace_dump ( DUMP_ALL ) ;
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} else if ( rcu_gp_in_progress ( ) & &
ULONG_CMP_GE ( j , js + RCU_STALL_RAT_DELAY ) & &
cmpxchg ( & rcu_state . jiffies_stall , js , jn ) = = js ) {
/* They had a few time units to dump stack, so complain. */
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print_other_cpu_stall ( gs2 , gps ) ;
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if ( READ_ONCE ( rcu_cpu_stall_ftrace_dump ) )
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rcu_ftrace_dump ( DUMP_ALL ) ;
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}
}
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//////////////////////////////////////////////////////////////////////////////
//
// RCU forward-progress mechanisms, including of callback invocation.
/*
* Show the state of the grace - period kthreads .
*/
void show_rcu_gp_kthreads ( void )
{
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unsigned long cbs = 0 ;
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int cpu ;
unsigned long j ;
unsigned long ja ;
unsigned long jr ;
unsigned long jw ;
struct rcu_data * rdp ;
struct rcu_node * rnp ;
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struct task_struct * t = READ_ONCE ( rcu_state . gp_kthread ) ;
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j = jiffies ;
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ja = j - data_race ( rcu_state . gp_activity ) ;
jr = j - data_race ( rcu_state . gp_req_activity ) ;
jw = j - data_race ( rcu_state . gp_wake_time ) ;
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pr_info ( " %s: wait state: %s(%d) ->state: %#lx delta ->gp_activity %lu ->gp_req_activity %lu ->gp_wake_time %lu ->gp_wake_seq %ld ->gp_seq %ld ->gp_seq_needed %ld ->gp_flags %#x \n " ,
rcu_state . name , gp_state_getname ( rcu_state . gp_state ) ,
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rcu_state . gp_state , t ? t - > state : 0x1ffffL ,
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ja , jr , jw , ( long ) data_race ( rcu_state . gp_wake_seq ) ,
( long ) data_race ( rcu_state . gp_seq ) ,
( long ) data_race ( rcu_get_root ( ) - > gp_seq_needed ) ,
data_race ( rcu_state . gp_flags ) ) ;
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rcu_for_each_node_breadth_first ( rnp ) {
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if ( ULONG_CMP_GE ( READ_ONCE ( rcu_state . gp_seq ) ,
READ_ONCE ( rnp - > gp_seq_needed ) ) )
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continue ;
pr_info ( " \t rcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld \n " ,
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rnp - > grplo , rnp - > grphi , ( long ) data_race ( rnp - > gp_seq ) ,
( long ) data_race ( rnp - > gp_seq_needed ) ) ;
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if ( ! rcu_is_leaf_node ( rnp ) )
continue ;
for_each_leaf_node_possible_cpu ( rnp , cpu ) {
rdp = per_cpu_ptr ( & rcu_data , cpu ) ;
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if ( READ_ONCE ( rdp - > gpwrap ) | |
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ULONG_CMP_GE ( READ_ONCE ( rcu_state . gp_seq ) ,
READ_ONCE ( rdp - > gp_seq_needed ) ) )
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continue ;
pr_info ( " \t cpu %d ->gp_seq_needed %ld \n " ,
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cpu , ( long ) data_race ( rdp - > gp_seq_needed ) ) ;
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}
}
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for_each_possible_cpu ( cpu ) {
rdp = per_cpu_ptr ( & rcu_data , cpu ) ;
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cbs + = data_race ( rdp - > n_cbs_invoked ) ;
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if ( rcu_segcblist_is_offloaded ( & rdp - > cblist ) )
show_rcu_nocb_state ( rdp ) ;
}
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pr_info ( " RCU callbacks invoked since boot: %lu \n " , cbs ) ;
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show_rcu_tasks_gp_kthreads ( ) ;
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}
EXPORT_SYMBOL_GPL ( show_rcu_gp_kthreads ) ;
/*
* This function checks for grace - period requests that fail to motivate
* RCU to come out of its idle mode .
*/
static void rcu_check_gp_start_stall ( struct rcu_node * rnp , struct rcu_data * rdp ,
const unsigned long gpssdelay )
{
unsigned long flags ;
unsigned long j ;
struct rcu_node * rnp_root = rcu_get_root ( ) ;
static atomic_t warned = ATOMIC_INIT ( 0 ) ;
if ( ! IS_ENABLED ( CONFIG_PROVE_RCU ) | | rcu_gp_in_progress ( ) | |
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ULONG_CMP_GE ( READ_ONCE ( rnp_root - > gp_seq ) ,
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READ_ONCE ( rnp_root - > gp_seq_needed ) ) | |
! smp_load_acquire ( & rcu_state . gp_kthread ) ) // Get stable kthread.
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return ;
j = jiffies ; /* Expensive access, and in common case don't get here. */
if ( time_before ( j , READ_ONCE ( rcu_state . gp_req_activity ) + gpssdelay ) | |
time_before ( j , READ_ONCE ( rcu_state . gp_activity ) + gpssdelay ) | |
atomic_read ( & warned ) )
return ;
raw_spin_lock_irqsave_rcu_node ( rnp , flags ) ;
j = jiffies ;
if ( rcu_gp_in_progress ( ) | |
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ULONG_CMP_GE ( READ_ONCE ( rnp_root - > gp_seq ) ,
READ_ONCE ( rnp_root - > gp_seq_needed ) ) | |
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time_before ( j , READ_ONCE ( rcu_state . gp_req_activity ) + gpssdelay ) | |
time_before ( j , READ_ONCE ( rcu_state . gp_activity ) + gpssdelay ) | |
atomic_read ( & warned ) ) {
raw_spin_unlock_irqrestore_rcu_node ( rnp , flags ) ;
return ;
}
/* Hold onto the leaf lock to make others see warned==1. */
if ( rnp_root ! = rnp )
raw_spin_lock_rcu_node ( rnp_root ) ; /* irqs already disabled. */
j = jiffies ;
if ( rcu_gp_in_progress ( ) | |
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ULONG_CMP_GE ( READ_ONCE ( rnp_root - > gp_seq ) ,
READ_ONCE ( rnp_root - > gp_seq_needed ) ) | |
time_before ( j , READ_ONCE ( rcu_state . gp_req_activity ) + gpssdelay ) | |
time_before ( j , READ_ONCE ( rcu_state . gp_activity ) + gpssdelay ) | |
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atomic_xchg ( & warned , 1 ) ) {
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if ( rnp_root ! = rnp )
/* irqs remain disabled. */
raw_spin_unlock_rcu_node ( rnp_root ) ;
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raw_spin_unlock_irqrestore_rcu_node ( rnp , flags ) ;
return ;
}
WARN_ON ( 1 ) ;
if ( rnp_root ! = rnp )
raw_spin_unlock_rcu_node ( rnp_root ) ;
raw_spin_unlock_irqrestore_rcu_node ( rnp , flags ) ;
show_rcu_gp_kthreads ( ) ;
}
/*
* Do a forward - progress check for rcutorture . This is normally invoked
* due to an OOM event . The argument " j " gives the time period during
* which rcutorture would like progress to have been made .
*/
void rcu_fwd_progress_check ( unsigned long j )
{
unsigned long cbs ;
int cpu ;
unsigned long max_cbs = 0 ;
int max_cpu = - 1 ;
struct rcu_data * rdp ;
if ( rcu_gp_in_progress ( ) ) {
pr_info ( " %s: GP age %lu jiffies \n " ,
__func__ , jiffies - rcu_state . gp_start ) ;
show_rcu_gp_kthreads ( ) ;
} else {
pr_info ( " %s: Last GP end %lu jiffies ago \n " ,
__func__ , jiffies - rcu_state . gp_end ) ;
preempt_disable ( ) ;
rdp = this_cpu_ptr ( & rcu_data ) ;
rcu_check_gp_start_stall ( rdp - > mynode , rdp , j ) ;
preempt_enable ( ) ;
}
for_each_possible_cpu ( cpu ) {
cbs = rcu_get_n_cbs_cpu ( cpu ) ;
if ( ! cbs )
continue ;
if ( max_cpu < 0 )
pr_info ( " %s: callbacks " , __func__ ) ;
pr_cont ( " %d: %lu " , cpu , cbs ) ;
if ( cbs < = max_cbs )
continue ;
max_cbs = cbs ;
max_cpu = cpu ;
}
if ( max_cpu > = 0 )
pr_cont ( " \n " ) ;
}
EXPORT_SYMBOL_GPL ( rcu_fwd_progress_check ) ;
/* Commandeer a sysrq key to dump RCU's tree. */
static bool sysrq_rcu ;
module_param ( sysrq_rcu , bool , 0444 ) ;
/* Dump grace-period-request information due to commandeered sysrq. */
static void sysrq_show_rcu ( int key )
{
show_rcu_gp_kthreads ( ) ;
}
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static const struct sysrq_key_op sysrq_rcudump_op = {
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. handler = sysrq_show_rcu ,
. help_msg = " show-rcu(y) " ,
. action_msg = " Show RCU tree " ,
. enable_mask = SYSRQ_ENABLE_DUMP ,
} ;
static int __init rcu_sysrq_init ( void )
{
if ( sysrq_rcu )
return register_sysrq_key ( ' y ' , & sysrq_rcudump_op ) ;
return 0 ;
}
early_initcall ( rcu_sysrq_init ) ;
